Women who have ever had breast cancer might want to walk away from the brie, the butter and the black cherry (and every other flavor) ice cream.
According to a study of 1 893 women, breast cancer survivors who average as little as one serving per day of high-fat dairy foods have a 49% higher risk of dying from breast cancer than those who eat little or no high-fat dairy.
Cancer survivors at risk of dying
In absolute terms, breast cancer survivors who consumed the most high-fat dairy had about a 12% risk of dying of the disease. The elevated mortality risk is therefore "modest," said lead author Candyce Kroenke, a staff scientist at Kaiser Permanente, the nonprofit healthcare provider.
"But since it may not be so difficult to lower your consumption of high-fat dairy, I think if you have breast cancer it's worthwhile."The research, published in the Journal of the National Cancer Institute, is notable because more than a dozen studies since the late 1980s have examined whether consuming milk, cheese, ice cream and other dairy products is related to breast cancer.
The results have been a confusing muddle: Some studies found that women who eat a lot of dairy have a higher risk of breast cancer, others found they have a lower risk and still others found no effect either way.
How the study was done
The Kaiser study is the first to separate out the effects of high- and low-fat dairy on women diagnosed with breast cancer. The hormone connection might apply beyond breast cancer. A 2012 study found that drinking more whole milk was associated with worse survival among men with prostate cancer, while skim milk was associated with higher survival.
"This is a very well-done study by highly regarded researchers," said Dr Michelle Holmes, associate professor of medicine and epidemiology at Harvard Medical School and Harvard School of Public Health, who was not involved in the research.
It advances scientists' understanding of how diet affects breast cancer, she said, and presents women with a simple dietary choice: "It's for each woman to decide, but if you don't eat high-fat dairy you can get the same nutrients from other sources," including low-fat versions.
Total dairy intake had no effect on how the women - who had been diagnosed with stage 1, 2 or 3A invasive breast cancer and most of whom were post-menopausal - fared over the 11.8 years, on average, that the researchers tracked them.
Higher rate of dairy intake
But high-fat dairy, which means whole milk or cream and anything made with them such as cheese and ice cream, did make a difference. Breast cancer survivors who ate one or more servings per day (according to a 120-item questionnaire they answered) also had a 64% greater risk of dying from all causes, but that was expected: A high-fat diet has long been associated with cardiovascular disease, among other illnesses.
The cancer risk was more surprising, if only because scientists have speculated that the vitamin D and calcium in milk might protect against cancer. Instead, the estrogens in milk might be the problem, researchers say. These hormones, which promote some breast cancers, reside in milk fat. Less milk fat means less oestrogens, so the oestrogen content of skim, 1% and 2% milk and products made from them is relatively low.
Another reason to suspect oestrogens rather than fat itself was that eating more saturated fat of all kinds did not raise the women's chances of dying of breast cancer as strongly as high-fat dairy did. That suggests that fat consumption per se is unrelated to breast-cancer mortality: nuts, chocolate, coconut and vegetable fats such as those in avocados did not increase the risk.